A problem that has drawn the attention of many inventors is that of slowing and/or stopping the rotation of a crane about its base through the use of hydraulic brakes. More generally, these same sort of brakes might be applied in any situation wherein a rotating shaft is to be braked and a metered dynamic braking response is desired. Broadly speaking, there are two sorts of brakes that are useful with machinery such as cranes. The first is a service or operating brake which is applied intermittently by the operator to control the crane's rotational motion while payloads are being moved. The second brake is a parking or emergency brake which is ordinarily engaged when the vehicle is left unoccupied. Additionally, some variations of the parking brake double as an emergency or fail-safe brake that engages when a hydraulic or other mechanical failure arises during operation. It is desirable in many circumstances that both types of brakes be available within the braking unit.
One of the most common braking systems encountered in the crane rotational braking arts is a hydraulically actuated disk-type brake, wherein a brake pack which contains interleaved stator disks (or just "stators") and rotators (or friction disks as they are also called) is compressed by one or more braking pistons. In those braking systems that offer both a parking and an operating type brake, it is customary to use either one or two pistons to accomplish these two purposes, single and dual pistons systems respectively, hereinafter.
In single piston braking systems, the operating and parking brake functions are performed by a single piston. It is well known to those skilled in the art that a single piston system is prone to give erratic braking results during service braking, at least until the operator "learns" a particular system. In more particular, when a single piston system is disengaged from "park," the braking piston must be moved some distance away from the braking media. When the operating brake is thereafter applied, the same piston must then travel an equivalent distance--in the opposite direction--to reengage the brake. From the operator's perspective, this can result in a brief period of time during which the brake pedal has no "resistance," followed by a sudden contact of the piston with the braking media and a resulting abrupt cessation of the crane rotation, i.e., overbraking. The typical single piston system might be described broadly as lacking a proportional braking feel, such as the feel one encounters in an automobile-type braking system. The ideal brake is one that has a firm feel, together with a braking responses that is proportional to the amount of pressure applied to the brake pedal.
Dual piston braking systems, on the other hand, have the potential to improve on the operating problem discussed previously by using separate operating and parking pistons. In theory at least, by using two braking pistons the service brake piston may be permanently positioned a distance away from the braking media that is a small fraction of the separation distance normally required for the parking brake piston. This has the potential to produce more uniform braking results, as the time required for the operating brake piston to move and engage the braking media can be made to be virtually imperceptible. From the operator's standpoint, this arrangement can result in a more consistent braking feel.
In some dual piston systems the parking brake contacts the brake pack only indirectly, e.g., though pressure on the operating brake piston--some have referred to this as a "series" arrangement. That is, the parking brake is applied by impelling the parking brake piston to push against the service brake piston, thereby moving the later to contact the braking pack--see e.g. Rach U.S. Pat. No. 3,974,896. Thus, in these sorts of systems the parking brake does not contact the braking pack directly, but only indirectly through the pressure on the top of the operating brake piston. An obvious disadvantage of these sorts of systems is that if the service brake becomes damages or unusable, that can also hamper or prevent the application of the parking brake.
The conventional wisdom seems to be that a dual piston system should have both pistons applied from the same side of the brake pack. Windish et al., U.S. Pat. No. 4,560,034 teaches that to do otherwise would require a greater amount of space and require having the entire friction disk-pack movable axially, the later factor apparently making this approach unpractical in his opinion. Others, e.g., Morgan et al. U.S. Pat. No. 4,263,991, have a dual piston system with the two pistons arranged on opposite sides of the brake pack, but wherein the actual braking force for both the parking and service brakes are applied from a single side of the brake pack.
However, dual piston systems wherein the pistons are applied from the same side of the brake pack have their own problems. The problems with a series-type two piston arrangement have already been discussed. In non-series systems, placing two pistons on the same side of the brake pack means that they must be interrelated in some fashion. For example, one possible arrangement would be to nest one piston inside of the other. However, this arrangement complicates the problem of separately activating the two pistons, as hydraulic pressure must somehow be independently brought to the inner piston. Nested piston systems are necessarily more complex mechanically, a factor which may decrease the reliability of the overall brake.
In systems that feature a parking brake it is not uncommon to design that brake to operate as a fail-safe device which is automatically applied in the absence of hydraulic pressure. These sorts of brakes are often spring actuated, the springs being designed to force the parking brake piston into the brake pack. (Typical spring constants for these sorts of springs might be in the neighborhood of 1000 pounds per inch of compression). Thus, in the absence of any outside forces, the parking brake will be forced automatically against the brake pack and the crane's rotation will be retarded. When the crane is operated, however, some provision must be made for disengaging the parking brake piston from contact with the brake pack. A common approach to doing this is through the introduction of a hydraulic force that is in opposition to the force exerted by the compressed springs. This lifts the parking brake piston away from the brake pack by further compressing the springs, thereby allowing the crane to rotate. If the hydraulics of the crane fail for any reason, the parking brake will automatically reengage and the crane's rotation will be stopped.
What is needed, then, is a two piston hydraulic braking system for use with cranes wherein the pistons may be independently actuated and which has proportional braking feel. Additionally, the system should be as simple as possible mechanically and fit within a typical crane rotation box.
Before proceeding to a description of the instant invention, however, it should be noted and remembered that the description of the invention which follows, together with the accompanying drawings, should not be construed as limiting the invention to the examples (or preferred embodiments) shown and described. This is so because those skilled in the art to which the invention pertains will be able to devise other forms of this invention within the ambit of the appended claims.